Why did the caterpillars cross the road?

A few days ago I was out walking the dog when we came across a bunch of caterpillars crossing the road. These were processionary caterpillars and are a familiar sight in this part of the world particularly at this time of the year. They walk in a line one after another, a habit that gives them their common name. They produce a trail of pheromones but it is the hairs at the rear of the caterpillar in front provide the main stimulus for the caterpillar following behind. Even though I have seen this behavior many times I had never stopped to consider WHY they should be marching in a procession like this away from the pine trees on which they grow over the winter.

So why DID the caterpillars cross the road?

The answer it seems is because they are now full-grown and preparing to find a suitable place to pupate. They will burrow down into the ground and undergo metamorphosis before appearing as winged moths some time between May and July.

All winter these caterpillars have been munching their way through pine needles during the hours of darkness, even during the coldest nights. They emerge from the nests they occupy during the day, again in procession, and crawl along the branches. Now that they are finished growing it is time to find a suitable place to pupate undisturbed. Interestingly, hoopoes are partial to the pupae if they happen to find them.

Even though these are pretty cool-looking things they are nasty little critters and should be avoided. If the hairs come into contact with skin they can produce a strong reaction. They are bad news for dogs too if the dog happens to sniff at them or, worse, take them into their mouths. A friend of Pippa nearly lost her dog when it did this and the dog´s tongue swelled up.

After I returned home with the dogs I went and found my camera and returned to take some photographs which are shown below.

Here is the whole procession on its way. Processions like this can be several metres long.

Head to toe

Here´s the lad in the front

Here is a typical nest. Interestingly there are no “doors” so when they caterpillars leave they just “bust out” through the silk.